Gas turbines having sequential combustion are known and have proved successful in industrial operation.
Such a gas turbine, which has become known in specialist circles as GT24/26, can be seen, for example, from the article by Joos, F. et al., “Field Experience of the Sequential Combustion System for the ABB GT24/GT26 Gas Turbine Family”, IGTI/ASME 98-GT-220, 1998 Stockholm. FIG. 1 there shows the basic construction of such a gas turbine, the FIG. 1 there being reproduced as FIG. 1 in the present application. Furthermore, such a gas turbine is apparent from EP-B1-0 620 362.
FIG. 1 shows a gas turbine 10 having sequential combustion, in which a compressor 11, a first combustion chamber 14, a high pressure turbine (HPT) 15, a second combustion chamber 17 and a low pressure turbine (LPT) 18 are arranged along an axis 19. The compressor 11 and the two turbines 15, 18 are part of a rotor which rotates about the axis 19. The compressor 11 draws in air and compresses it. The compressed air flows into a plenum and from there into premix burners, where this air is mixed with at least one fuel, at least fuel fed via the fuel supply 12. Such premix burners are apparent in principle from EP-A1-0 321 809 or EP-A2-0 704 657.
The compressed air flows into the premix burners, where the mixing, as stated above, takes place with at least one fuel. This fuel/air mixture then flows into the first combustion chamber 14, into which this mixture passes for the combustion while forming a stable flame front. The hot gas thus provided is partly expanded in the adjoining high pressure turbine 15 to perform work and then flows into the second combustion chamber 17, where a further fuel supply 16 takes place. Due to the high temperatures which the hot gas partly expanded in the high pressure turbine 15 still has, a combustion which is based on self-ignition takes place in the combustion chamber 17. The hot gas re-heated in the second combustion chamber 17 is then expanded in a multistage low pressure turbine 18.
The low pressure turbine 18 comprises a plurality of moving blades and guide blades which are arranged alternately one behind the other in the direction of flow. The guide blades of the third guide blade row in the direction of flow are provided with the designation 20′ in FIG. 1.
At the high hot gas temperatures prevailing in gas turbines of the newer generation, it has become essential to cool the guide and moving blades of the turbine in a sustainable manner. To this end, a gaseous cooling medium (e.g. compressed air) is branched off from the compressor of the gas turbine or steam is supplied. In all cases, the cooling medium is passed through cooling channels formed in the blade (and often running in serpentine shapes) and/or is directed outward through appropriate openings (holes, slots) at various points of the blade in order to form a cooling film in particular on the outer side of the blade (film cooling). An example of such a cooled blade is shown in publication U.S. Pat. No. 5,813,835.
The guide blades 20′ in the known gas turbine from FIG. 1 are designed as cooled blades which have cooling channels running in the interior in the radial direction, as have become known, for example, from publication WO-A1-2006029983. Such guide blades are produced with the aid of a high-tech casting process, wherein the casting material is fed from both sides (inner platform and outer platform) of the casting mold. On account of the comparatively thin walls of the airfoil and on account of the channels and openings produced for the cooling air during the casting process, the service life, the cooling air consumption and the cooling effect achieved greatly depend on the precision that can be achieved during the casting process. This is especially the case when such blades also have a pronounced spatial curvature.